This invention relates to the autocatalytic plating of metallic films on substrates, and in particular to an improved process for increasing the deposition rate of the films on the substrate without affecting the quality of the deposited films.
In autocatalytic plating (also referred to as electroless plating or deposition) a chemical reducing agent in solution reduces metallic ions to a metal which is deposited on a suitable substrate. The plating takes place only on "catalytic" surfaces rather than throughout the solution. The catalyst is initially the substrate, and subsequently the metal which is deposited on the substrate.
Electroless plating is a well known technique for the plating of nickel-phosphorus alloys. A typical plating bath for the electroless deposition of nickel-phosphorus includes a nickel salt, a reducing agent such as sodium hypophosphate (NaH.sub.2 PO.sub.2), a complexing agent to help keep the nickel in solution and a compound which increases the stability of the bath. The deposition rate of nickel-phosphorus on the substrate is a function of, among other things, the pH and the operating temperature of the bath. While it is desired to operate the bath at as high a temperature as possible, localized boiling within the bath profoundly disrupts the transport of the nickel to the substrate, resulting in unacceptable film properties. In addition, localized boiling causes precipitation of nickel within the bath which can result in spontaneous decomposition. Certain types of materials (referred to as exaltants) increase the deposition rate without increasing the operating temperature of the bath. The mechanism by which they speed up deposition has not been explained completely.
A detailed description of electroless nickel-phosphorus deposition is found in Symposium on Electroless Nickel Plating, ASTM Special Technical Publication No. 265, 1959, and Thin Film Processes, Vossen, John L., Ed. and Kern, Werner, Ed., Academic Press, 1978, pp. 213-218.